The present invention relates generally to the manufacture of display screens for cathode-ray tubes, and more particularly to an improved method for making multicolor CRT display screens having minimal phosphor contamination. The invention has particular utility in the production of phosphor dot screens for high resolution color display tubes of the shadow mask type, and especially to "black surround" versions of them. For that reason, the invention will be described primarily with reference to the manufacture of such screens.
A conventional dot screen type color display tube includes three electron guns arranged in a delta configuration. The three guns project a like number of electron beams through a shadow mask onto a display screen comprising a mosaic pattern of phosphor deposits arranged in a multiplicity of dot triads. Each triad includes a dot of a red-, a green-, and a blue-emitting phosphor. For improved display brightness, the screen may include a matrix layer of light-absorbing material that surrounds and separates the phosphor dot deposits. Such a screen, which has come to be known as a "black surround" screen, is the subject of U.S. Pat. No. 3,146,368 to Fiore et al.
The mosaic phosphor dot pattern of a dot-screen tube normally is formed by a direct photoprinting process that proceeds substantially as follows: A screen area on the inner surface of the faceplate is first coated with a photosensitive slurry containing phosphor particles of one color. Next, the tube's shadow mask is mounted on the faceplate temporarily and the coating is exposed to ultravoilet light projected through the mask apertures from a source located at a position corresponding to that of the related electron gun. The shadow mask is then removed and the coating treated to remove the unexposed portions, leaving a pattern of dots of one phosphor color. These steps are repeated for each remaining color to deposit a triangular group of three phosphor dots--a red, a green, and a blue--on the faceplate opposite each mask aperture.
Normal practice is to make the individual phosphor dots smaller in size than the corresponding mask apertures. This is generally accomplished by exposing the dots through a shadow mask that has apertures of a temporarily smaller size. Then, after the phosphor dots are deposited, the mask is re-etched to enlarge the apertures to a final, larger size. Re-etching of shadow mask apertures is shown in U.S. Pat. No. 2,961,313 to Amdursky, for example. An alternative procedure is to reduce the diameter of the shadow mask holes temporarily by electroplating, as described in U.S. Pat. No. 3,231,380 to Law, or by electrophoretic coating with a non-metallic material, as taught by U.S. Pat. No. 3,070,441 to Schwartz. The size of the phosphor dots also can be made smaller without modifying the shadow mask by very careful control of the light exposure step. See, for example, previously mentioned U.S. Pat. No. 3,146,368.
Black surround screens have been made in a variety of ways, but the usual procedure is to form the light-absorbing matrix layer before depositing the phosphor dots. For example, as described in U.S. Pat. No. 3,558,310 to Mayaud, the screen area of the faceplate is coated first with a photohardenable material, such as dichromate-sensitized polyvinyl alcohol (pva). With the shadow mask mounted in position, the coating is given three separate exposures, one from each electron gun position. The mask is then removed and the unexposed portions of the coating washed off, leaving a pattern of hardened pva dots. The dot pattern is covered with a light-absorbing coating of colloidal graphite, which is dried and then treated with a chemical agent, such as hydrogen peroxide, to remove the pva dots and the overlying portions of the graphite coating. This provides the screen area with a light-absorbing matrix layer having a pattern of openings for receiving the color phosphor dots, which are then deposited as previously described.
Screening methods of the prior art as described have a number of limitations, particularly when applied to the production of high resolution display screens and those requiring superior color quality. For example, to build a color CRT with double the resolution of a standard entertainment-type tube, phosphor dot density must be increased by a factor of four and dot size reduced accordingly. Decreasing phosphor dot size is not simply a matter of using a shadow mask with smaller apertures, however, Diffraction effects make it necessary to decrease aperture size more than a factor of two to achieve a 2.times. reduction in dot size using standard screening methods. Moreover, these effects increase with decreasing aperture size, so that it would be necessary to use extremely small apertures to facricate a screen with dots one-quarter the diameter of those in an equivalent size entertainment TV tube. The resulting dots would tend to be irregular and nonuniform in diameter because of the effects of edge diffraction on the pva exposure process.
Another limitation of prior art processes applicable to normal and high resolution screens alike is contamination of the various color phosphor deposits during processing. For example, as pointed out in U.S. Pat. No. 3,615,462 to Szegho et al., the phosphor dots may become contaminated by particles of light-absorbing pigment released from the black surround layer during the phosphor screening steps. That patent proposes the use of a clear, volatilizable protective overcoating on the pigmented layer to overcome the problem. While perhaps effective for the intended purpose (at a cost of increased process complexity) it does nothing to prevent cross-contamination of the phosphors themselves, a more serious problem when color purity is a paramount objective.
Still another limitation of conventional screening processes is that the phosphor dots are formed by a "front" exposure--i.e., by light directed onto the free surface of a photosensitive layer. Because the photopolymerization process begins at the side of the layer nearest the light source and proceeds through the layer to the support as the exposure continues, exposure and coating uniformity must be tightly controlled if well-adhered dots of uniform size are to be obtained. Underexposure or an overly-thick photosensitive layer may produce dots that fail to adhere to the faceplate. Overexposure (or a too-thin layer) causes overly-large dots with ragged edges. Light diffraction produced by exposure through small mask apertures greatly increases these problems. Thus, the resolution of large screen (.gtoreq.12 in. dia.) CRT screens is limited as a practical matter by conventional process technology to matrixes composed of 0.006 in. dia. dots on 0.012 in. centers.
A general object of the present invention is, therefore, to provide an improved color display screening process that is free of the drawbacks enumerated above.
A more specific object of the invention is to provide a novel method for applying a pattern of uniform, well defined deposits on the faceplate of a cathode-ray tube.
Another significant object of the invention is to provide a method for screening shadow mask multicolor display CRT's that minimizes the possibility of contaminating the phosphor deposits.
Still another object of the invention is to provide a method for manufacturing very high resolution multicolor CRT display screens.
A further object of the invention is to provide an improved screening method utilizing a through-the-faceplate exposure to form multicolor phosphor deposits in a black surround screen.